public class Demo1 {
    public static int count = 0;
    public static void main(String[] args) {
        Object locker = new Object();
        Thread t1 = new Thread(() -> {
            for(int i = 0; i < 10000; i++){
                synchronized(locker){
                    count++;
                }
            }
        });
        Thread t2 = new Thread(() -> {
            for(int i = 0; i < 10000; i++){
                synchronized(locker){
                    count++;
                }
            }
        });
        t1.start();
        t2.start();
        try {
            t1.join();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        try {
            t2.join();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        System.out.println("count = " + count);
    }
}